Membrane trafficking pathways play central roles in cell physiology, including in responses to environmental challenges like infection with microorganisms. Some pathogens are destroyed by these pathways, while others subvert their function. Toxoplasma gondii is an obligate intracellular parasite that multiplies in the cytoplasm of mammalian cells within a self-made membrane-bound compartment - the parasitophorous vacuole (PV). The PV membrane has a unique lipid and protein composition, and the PV does not fuse with any endocytic or exocytic organelles. However, we showed that the parasite's intracellular survival relies on nutrients present in mammalian organelles. For example, T. gondii retrieves cholesterol and sphingolipids from endocytic organelles and Golgi vesicles, respectively, which raises the perplexing question of how T. gondii can access the lipid content of these organelles without fusion. To address this issue, we analyzed vesicular trafficking pathways in infected mammalian cells. T. gondii targets several host pathways and intercepts the mammalian traffic mediated by Rab recycling and secretory vesicles. All of these Rab vesicles are re-routed to the PV and then sequestered intact in the vacuolar lumen. Golgi Rab vesicles trapped in the PV contain sphingolipids that are salvaged by the parasite. Our hypothesis is that T. gondii acquires the needed lipids by macroendocytosis of nutrient-filled vesicles into the PV. The goals of this application will be to unravel the complexity of this process in mechanistic detail and identify future targets for intervention. We will conduct genetic cell biological and biochemical approaches to characterize the interactions of the PV with mammalian Rab GTPases and their effectors; to identify the parasite and mammalian proteins that mediate the transport of Rab vesicles to and across the PV membrane; and to analyze the fate of the scavenged lipids in the parasite by characterizing the function of lipid transporters. . gondii can cause fatal encephalitis in immunocompromised individuals, and current treatment options for toxoplasmosis are limited and poorly tolerated. Rab GTPases and Rab-regulated pathways are important targets in human disease, yet are underexplored as therapeutic targets. Using Rab GTPAses as markers to understand the process of host vesicle uptake by T. gondii, we expect to identify key factors usurped from the host cell and/or expressed by T. gondii to expose new vulnerabilities for the parasite. Studying the mechanisms used by this parasite to control Rab-mediated vesicle trafficking may yield valuable insights into how these GTPases coordinate membrane transport in healthy cells.